1. Field of the Invention
The present invention relates to proportional solenoid valves and, more particularly, to a proportional solenoid valve which utilizes an elongated cup armature and non-magnetic seat located near the region of highest flux density, and provides zero bias, that is, substantially zero differential pressure when no current is present in the electric winding of the solenoid.
2. Description of the Prior Art
A variety of solenoid valves have been developed in which electromagnetics are utilized in an attempt to control either hydraulic or pneumatic flow. Typically, such valves attempt to regulate the flow of fluid in an on-off fashion. Where modulation is desired, two valves are frequently used in a timed, alternating operation to provide an output which is a function of the timing and the two inputs.
This same type of modulation has also been accomplished with a single device where the armature of the valve actuated between two oppositely disposed seats to permit a mixture of fluid at atmospheric pressure and fluid at a higher or lower pressure. In such a valve, a large mass iron armature is spring-biased against a first seat and utilized to prevent fluid flow through the seat. When in this position, fluid at atmospheric pressure is allowed to enter the valve through a second seat. At appropriate times, current is supplied to an electromagnetic winding and the spring force against the armature is overcome by the magnetic flux and the armature traverses an air gap until it contacts the second seat, located on the opposite side of the valve. When the armature contacts the second seat, fluid at atmospheric pressure is prevented from entering the valve through the second seat. Fluid at either a high pressure (above atmospheric) or a vacuum (below atmospheric) source is permitted to enter the valve through the first seat.
At a later time in the cycle, the current is reduced in the winding and the solenoid is de-energized, allowing the spring force to overcome any hysteresis effects on the armature and cause the armature to travel back across the air gap to the first seat. This prevents flow from the high pressure fluid source or the vacuum fluid source through the first seat and allows fluid at atmospheric pressure to enter the valve through the second seat. In this on-off fashion, the armature alternatively and periodically allows fluid to enter the valve through the first seat and the second seat. By applying a controlled duty cycle to alternately open and close the inlets through the first and second seats, the valve attempts to provide the approximation of a desired outlet pressure. Such valves, however, are not able to provide adequate speed or control of the output pressure through this on-off electronic adjustability and fail to provide a linear relationship between the current input to the winding and the differential pressure output from the valve. Such systems often require feed-back or closed loop operation.
Some of these prior art valves utilize complex assemblies that operate in conjunction with ball valves. Other types of these prior art valves utilize flapper-type valves that attempt to balance the electromagnetics against fluid blast or fluid flow.
Many of the disadvantages and complexities of these prior art solenoid valves were eliminated with the development of the proportional solenoid valves of U.S. Pat. Nos. 4,534,375 and 4,715,396. The development of the aforementioned proportional solenoid valves provided a mechanism for modulating the differential pressure in a chamber by varying the energization level of the electrical winding in an electromagnetic assembly between a vacuum source and ambient. This valve included a restriction in the fluid conduit between the vacuum source and the differential pressure chamber. Such a restriction substantially reduced the fluid flow required for actuation. The large mass armature of the prior art valves was replaced by a low-mass armature which provided quick response and resulted in proportional control and a near linear relationship between current input to the electrical winding and output differential pressure.
The proportional solenoid valve development also provided a valve which allowed adjustment of one of the segments in a series of segments in a magnetic flux circuit. The adjustment of a segment allowed a variation of the set point of the current versus differential pressure curve. The result was greatly improved control over the current versus differential pressure relationship. It also generally eliminated any requirement of feed-back or closed loop operation for calibration.
The zero bias proportional solenoid valve of the present invention provides a new approach to proportional solenoid valves. The valve of the present invention provides a valve having quick response and repeatability by locating the non-magnetic seat nearer to the point of highest flux density but between the point of highest flux density and the armature. Moreover, the utilization of an elongated armature with a flat disc member at its base, or a flat portion to contact the opening to the vacuum source, provides a more efficient valve with a zero bias, that is, the substantial elimination of leakage of fluid from the vacuum source when the valve is without current in the electric winding of the solenoid.